This invention relates to the control of fouling by mollusks in aqueous systems by utilizing a nitrostyrene compound and an alkyl thiocyanate compound.
More particularly, this invention relates to control of potential mollusk fouling in cooling systems for both industrial plants and utilities which are subject to such fouling, whether the system is using cooling water on a once-through basis or is of the recirculating type. The once-through systems operate by drawing cooling water through the process to be cooled on a one-time basis and discharge the water directly to the receiving body with a short residence time (usually minutes to hours), whereas recirculating cooling systems require the addition of only a fraction of the system volume as make-up water. Additionally, the service water systems (waste, safety and auxiliary cooling) which are often a part of these cooling systems are also quite vulnerable to mollusk fouling, primarily because they do not run continuously, the conduits are of a smaller diameter, and more time is allowed for mollusk growth.
Mollusks are among the broad range of macroinvertebrates which can cause macrofouling problems to marine and fresh water cooling systems. Macrofouling by mollusks, like other groups of macrofouling macroinvertebrates--barnacles, bryozoans, sponges, hydroids, tunicates and annelids--is initiated by the settlement or attachment of larval and/or juvenile stages that are easily entrained by the service waters of cooling systems.
Fouling caused by the settlement, attachment and/or biogrowth of the mollusks in the cooling systems and associated service water systems of the industrial plants and utilities which utilize large quantities of water is a major problem causing a variety of deleterious effects to the structure, operation and safety of these systems. As indicated in the U.S. Nuclear Regulatory Commission 1984 Report entitled "Bivalve Fouling of Nuclear Power Plant Service--Water Systems", the safe operation of a nuclear power plant is a concern because of fouling caused by the Asiatic clam (Corbicula fluminea), the blue mussel (Mytilous edulis) and the American oyster (Crassostrea virginica). This report describes the correlations between the biology of these bivalve mollusks and the design and operation of power plants that allow bivalves to enter and reside within their cooling water systems.
The extent and type of mollusk fouling will depend upon many factors such as the source of the cooling water, the season, the water temperature, the growth rate of the fouling mollusk, and the linear velocity of the cooling water. Because of the large quantities of cooling water used, the locality of the plant will dictate the water's source. A fresh water cooling system will be drawing from a river, lake or well, whereas plants situated along coastal areas will most likely utilize brackish or marine water for their systems.
Both once-through and recirculating types of cooling water are treated prior to entering the system by screening to remove objects which are large enough that they could damage pumps and heat exchange equipment. This screening does not prevent the passage of the early life-stages or larval stages of the mollusks, which are the precursors to fouling as growth conditions are usually favorable within these systems. These early life stages of the mollusks will settle out in low flow areas or attach to substrate within the cooling system and grow to mature organisms.
One of the species of mollusks controlled by the method of this invention is the Asiatic clam, Corbicula spp. As indicated in the article entitled "Freshwater Macrofouling and Control with Emphasis on Corbicula" in the December 1983 Proceedings of the Electric Power Research Institute (EPRI), the Asiatic clam has caused significant incidents of macrofouling to fresh water cooling systems of power plants. Another freshwater mollusk, Dreissena spp--the Zebra mussel, causes fouling problems in Europe to cooling systems in a similar manner as the Asiatic clam. Both Drissena and Corbicula have free floating planktonic veliger larvae which allow easy penetration into cooling systems. Similar macrofouling problems plague cooling systems using estuarine or marine waters, but with different species of mollusks.
As a specific example of how a mollusk can cause fouling problems, a description of some characteristics of the Asiatic clam follows:
One-year-old clams are capable of plugging valves and nozzles. Two-year-old clams can cause mechanical damage to impellers and other moving parts of water-distribution systems. At six years, the clam can damage tires of construction vehicles. As in all other clams, growth is rapid in early years and then tapers off. "Clams--A Growing Threat to Inplant Water Systems", Plant Engineering, June, 1979, p. 165.
The Asiatic clams are very tolerant of many chemicals and often occur in great abundance. They have accumulated to depths of two meters in the Delta-Mendota Canal in California and have caused reduction in water flow. Some industrial plants have had difficulty obtaining fire insurance after inspectors found the fire protection systems plugged with Corbicula shells. Pump impellers have been damaged by shells in some industrial plants. The number of power plants which have experienced problems with this species has been steadily increasing during the past several years. Problems in fossil-fueled power plants most often relate to pluggage of condenser tubes, surface water heat exchangers, and blockage of fire protection systems. In addition to these problems, nuclear power plants may have other problems associated with the shutdown service water, and emergency reactor cooling systems.
For further information, see also Mattice, J. S., 1979. "Interactions of Corbicula sp with Power Plants", pages 119-138 and Goss, L. B. et al., 1979, "Control Studies on Corbicula for Steam Electric Generating Plants", pages 139-151, in J. C. Britton (ed), Proceedings, First International Corbicula Symposium, Texas Christian University Research Foundation, Fort Worth, Texas, 313 pages.
Fouling control of mollusks has been attempted using physical/mechanical and chemical techniques, but no foolproof combination has been developed. For example, chlorine, which has been by far the most used biofouling control agent, has several limitations: prolonged exposures are required to achieve efficacy, chlorine demand of the cooling water reduces its potency, and strict environmental regulations are being imposed which act to severely limit the discharge of chlorine residues, and in some cases seek to eliminate its use altogether.
In addition to chlorine, Smith, U.S. Pat. No. 4,462,914 discloses the use of a high density cationic polymer to control Corbicula. While the polymer appears to be efficacious toward the adult clam after a six day exposure period, it suffers from some of the same drawbacks as chlorine.
The above-mentioned concerns over potential fouling biocides is well described by the following excerpt from the December 1983 Proceedings of the Electric Power Research Institute:
"Chemical controls have an inherent liability. What can kill inside the power plant may also impact the receiving water body: chemical toxicants are not specific. The perfect chemical would be stable enough to be effective inside the plant, but become non-toxic, via chemical reaction or decay, before or as it entered the receiving water body. So far, no chemical meets these specifications: chlorine and bisulfate/sulfide which have actually been used in an attempt to control Corbicula fouling have not been effective alone or have been successful only under limited conditions. Such a chemical may not exist, but scheduling of application of a chemical at the beginning of scheduled outages may offer a less stringent alternative, because of the possibility of extending holdup times".
The compounds of the present invention have been utilized for control of microorganisms, i.e., bacteria, fungi, and algae, but have not been suggested for control of larger, more complex organisms, especially mollusks. See, e.g., U.S. Pat. No. 3,898,343. Bacteria, fungi and algae microorganisms are dependent upon the presence of metabolizable components in an aqueous system. However, the presence or absence of mollusks is essentially independent of the presence of metabolizable components in the water because they are a much more complex organism than the microorganisms, both in terms of anatomic and physiological complexity and position in the food chain. Mollusks are unable to exist on metabolizable components. Rather, mollusks require small plants or animals as foodstuff. Until the unexpected discovery of the applicants, the use of the compounds of the present invention have never before been appreciated to control mollusks.